Speculative Precision Time Protocol: Submicrosecond clock synchronization for the IoT

Time synchronization is a keystone of Wireless Sensor Networks (WSN). It is fundamental to coordinate the action of nodes in a network and it is also a critical element of several security mechanisms. In this paper, we discuss and evaluate the time synchronization strategy behind the Trustful Space-Time Protocol (TSTP), which explores the protocol's cross-layer architecture to speculatively peek through the timestamps and geographic info present in message headers, implementing high-accuracy clock synchronization with minimal insertion of explicit messages. We evaluate the protocol analytically and experimentally. The analytic evaluation is based on the model defined by Schmid [15] for the Virtual High-resolution Time (VHT), while the experimental evaluation was performed on the IEEE 802.15.4-compliant EPOSMote platform running EPOS and TSTP. Our results demonstrate that nodes in the network can be consistently synchronized with sub-microsecond precision while exchanging far less messages than they would with an ordinary, non-speculative implementation, resulting in energy savings. Indeed, precision and energy savings are higher for networks with higher traffic, since more messages are available for peeking. In an experiment scenario in which messages were exchanged between devices every 15 seconds, nodes in the network achieved a synchronization error of approximately 15 microseconds in the worst case, while in a scenario in which messages were exchanged every 3 seconds, synchronization error was less than 0.5 microseconds in the worst case, and approximately 0.25 microseconds on average.